1. Field of the Invention
The present invention relates to a method and system for achieving maximum regenerative braking capability for a vehicle having both regenerative and friction braking subsystems, while maintaining transparency of the transition between friction braking and regenerative braking following an antilock brake system (ABS) event.
2. Disclosure Information
Electrodrive vehicles have been designed, and built for many years. Regenerative braking allows such vehicles to present the attractive possibility of salvaging and transforming into potential energy, kinetic energy which would otherwise be converted into thermal energy in a friction braking system. Nevertheless, it is common with current vehicles having electrodrive capability to maintain both regenerative and friction braking subsystems, because of the inherent advantage provided by having both types of brakes available on the vehicle. Thus, it is known to use friction brakes to achieve ABS capability, while employing regenerative braking to a greater or lesser extent when the ABS is not in operation. U.S. Pat. No. 6,122,588, although showing the combination of regenerative braking with friction braking, does not disclose or teach any system or method for handling the regenerative braking in conjunction with ABS provided by a friction braking subsystem.
It is important to handle the interface between the friction braking subsystem and the regenerative braking subsystem properly, particularly within the moments following an ABS event. Because operation during ABS is usually accomplished with concomitant disablement of the regenerative braking subsystem, it is necessary to re-introduce the regenerative braking subsystem at the conclusion of an ABS event. This re-introduction of regenerative braking must be handled correctly so as not to trigger a new ABS event. A system and method according to the present invention is intended to allow regenerative braking to be re-introduced to the maximum possible extent without causing either additional ABS events or objectionable brake torque fluctuations.
It has been determined by the present inventors that when operating a combined braking system having regenerative and friction braking subsystems, it is also important that the regenerative braking limit, which is the contemporaneous maximum regenerative brake torque, taking into consideration the regenerative system limitations and road conditions, be increased only in conjunction with balanced braking. It is of course undesirable to attempt to apply the regenerative brakes at a torque in excess of the torque which may be sustained by the surface upon which the vehicle is being operated. Thus, if the driver demands a brake torque in excess of that achievable by the regenerative braking subsystem, friction braking will be employed to achieve the desired level of braking. This method of operation will achieve the maximum regenerative effect without causing needless ABS events.
A combined regenerative and friction braking system for the road wheels of a vehicle includes a friction braking subsystem, a regenerative braking subsystem coupled to the road wheels equipped with the friction braking subsystem, and a brake system controller which operates the braking subsystems in an antilock mode, as required. The brake system controller controls and operates both the friction braking subsystem and the regenerative braking subsystem such that operation in the antilock operating mode will be followed by a period of operation in which the maximum braking torque applied by the regenerative braking subsystem to a particular axle is limited to a level comparable to the maximum braking torque applied by the friction braking subsystem as well as by the regenerative braking system as a period of antilock operation ends. The brake system controller operates the regenerative braking subsystem so as to restore regenerative braking torque up to a maximum value as quickly as possible. In essence, the regenerative braking torque limit is ratcheted up to a maximum value in accord with the total brake torque generated by the regenerative and friction brakes on the axle.
The regenerative braking subsystem may comprise a rotating electrical machine coupled to at least one road wheel as well as to an electrical storage device. Alternatively, a regenerative braking subsystem may comprise a hydraulic pump/motor coupled to at least one road wheel as well as to a hydraulic energy storage device. Other types of regenerative braking are suitable for use with a system according to the present invention, including kinetic energy storage devices. What is important is that the vehicle have a means for coupling at least one road wheel to an energy storage device so that kinetic energy in the vehicle may be captured as potential energy in the energy storage device whenever braking occurs.
According to another aspect of the present invention, a brake system controller operates a friction braking subsystem and a regenerative braking subsystem such that when antilock operation ceases, brake torque will initially be applied by both the regenerative and friction braking subsystems, with the controller transitioning to a state in which all of the demanded brake torque is applied by the regenerative braking system whenever the balanced brake torque is less than a maximum system value.
A system and method according to present invention may be applied to each of the road wheels of a vehicle. That is, each of the road wheels may be integrated into a friction braking and regenerative braking subsystem according to the present invention.
According to another aspect of the present invention, a method for operating friction braking and regenerative braking subsystems includes the steps of operating either solely the friction braking subsystem, or both the friction and regenerative braking subsystems in an antilock mode, and transitioning from an antilock mode to a mode in which the fraction of demanded braking torque provided by the friction braking subsystem is decreased while the fraction of demanded braking torque provided by the regenerative braking system is increased.
The maximum allowable brake torque developed by the regenerative system while operating in the maximum regeneration mode is the lesser of the maximum torque capability of the regenerative braking system and the balanced braking torque demanded by the driver of the vehicle.
According to another aspect of present invention, the contemporaneous maximum permissible torque for the regenerative braking system is based upon at least the amount of time which has elapsed since the cessation of the latest period of operation in antilock mode. Further, if the vehicle""s operator further depresses the accelerator, the maximum torque capability of the system will be reinstated. This will be the maximum system torque. If the net torque requested by the driver becomes a drive torque rather than a brake torque, the maximum regenerative capability is restored.